This invention relates to a helmet display system and in particular to a helmet display system for military aircraft, which display system can be used in conjunction with a head-up display (HUD).
It is desirable to present information to military aircrew in a visible form within the direct field of view of the aircrew. Such information may relate to the altitude, speed, etc of the aircraft, information from a radar, or may be a sight by which weapons can be targeted. Such information is conventionally provided by a head-up display which is fixed relative to the aircraft. Because of this, this information may not be in the direct line of sight of a member of the aircrew if that member is looking other than in the straight ahead position. Also, if a target sight is displayed then the aircraft will have to be aligned with the target. More recently, helmet display systems have been developed whereby the desired information is provided on the visor of a helmet. This is achieved by providing a combiner on the visor, which reflects radiation enabling an image projected on the rear surface of the visor to be seen by the wearer of the helmet.
Holographic combiners can be frequency tuned to the selected waveband of a projected image, and give good transmission values at frequencies outside that band, but unfortunately they also reflect radiation in the selected waveband which is received externally of the visor. This is particularly a problem where the helmet display is to be used in conjunction with a conventional head-up display, for the waveband of both the helmet display and head-up display are likely to be the same, normally about 545 nm, as at this wavelength the eye is particularly sensitive. Therefore a large proportion, typically about 80%, of the radiation from the head-up display incident on the combiner is reflected.
According to the present invention there is provided a helmet display system comprising a display visor including a combiner reflective to radiation within a selected relatively narrow waveband of the visible spectrum; means for projecting an image within the selected waveband to an eye of a wearer of the helmet by reflection from the combiner; and an anti-glare visor having high transmission values in a waveband of the visible spectrum substantially corresponding to the selected waveband relative to the transmission values for the remainder of the visible spectrum, the display and anti-glare visor being arranged such that in use the helmet wearer may view a scene through both visors simultaneously.
The present invention arises from the realisation that the head-up display of an aircraft, along with other instrumentation, is designed to be capable of providing a display at a sufficiently high intensity such that it can be viewed with a neutral density (anti-glare) visor deployed, which typically has a transmission value of approximately 16 percent. However, when a holographic combiner is incorporated on a display visor, then this combiner will attenuate the signal within the particular waveband to which it is tuned, and therefore the transmission value of the anti-glare visor within the selected waveband can be increased. This then compensates for attenuation by the combiner, and preferably the sum of the transmission values of the two visors are substantially uniform over the visible spectrum. Therefore by employing the present invention the wearer of the helmet can see both the helmet display and a conventional head-up display, even when the anti-glare visor is deployed. Furthermore the intensity of the head-up display does not need to be adjusted dependent on the position of the anti-glare visor, and the colour cast obtained when using the display visor by itself, or in combination with a conventional neutral density visor (due to the attenuation of the selected waveband by the combiner), is degraded or removed altogether when the visors of the present invention are used in combinations.
The glare visor may advantageously be photochromic.